1. Field of the Invention
The present invention relates to a wire bonding method and apparatus.
2. Prior Art
As universally known, in a wire bonding apparatus, a bonding head is mounted on an XY table, and this XY table is driven in the directions of the X and Y axes (two perpendicular directions on the horizontal plane) by means of an XY-axis motor (hereafter referred to as XY-axis driving). An ultrasonic horn which is raised and lowered or pivoted upward and downward by a Z-axis motor (hereafter referred to as Z-axis driving) is disposed on the bonding head with a bonding tool attached to one end of the ultrasonic horn. A wire wound on a wire spool passes through the bonding tool via a clamper.
Wire bonding is performed at the bonding timing shown in FIG. 4.
In particular, as a result of horizontal-plane movement of the bonding head (which makes a horizontal-plane movement of the bonding tool) by the X-Y-axis driving of the XY table and lowering movement of the ultrasonic horn by the Z-axis driving, the bonding tool is lowered so that a ball formed on the tip of the wire contacts the first bonding point 1. The bonding tool is then slightly lowered even further so that the ball at the tip of the wire is pressed against the first bonding point 1. While the ball is being pressed, the ultrasonic horn performs an ultrasonic oscillation so that an ultrasonic vibration is applied to the bonding tool, thus bonding the ball to the first bonding point 1.
Next, the bonding tool is raised to the height level C, which is vertically an intermediate point, driven in the directions of the X and Y axes and lowered, delivering the wire. The wire is then brought so as to contact the second bonding point 2, and ultrasonic vibration and the application of pressure to a portion of the wire are performed in the same manner as-done in the step of bonding of the first bonding point 1, thus bonding the wire to the second bonding point 2. Afterward, the bonding tool is raised, and the damper is closed during this raising process of the bonding tool, thus cutting the wire from the second bonding point 2.
Conventionally, in the raising operation of the bonding tool to an intermediate position C after the bonding to the first bonding point 1, the Z-position of the bonding tool at the time that the bonding tool bonds the ball to the first bonding point 1 is used as a reference position, and the intermediate position C is calculated by performing mathematical operations on this reference position and the raising amount of the bonding tool based on looping parameters stored in the memory beforehand. Afterward, a looping operation is performed by the XY-axis driving simultaneously with the raising of the bonding tool by the Z-axis driving.
Wire bonding methods described above are described in, for example, Japanese Patent Application Publication (Kokoku) No. H1-31695 and Japanese Patent No. 2530224.
However, in semiconductor devices, the amount of sinking-in caused by the pressing force of the bonding tool is not always necessarily uniform, and it differs from bonding point to bonding point. Furthermore, when the bonding tool is raised for looping (or loop formation) so that the pressing of the bonding tool is eliminated, the bonding tool may return to the pre-sinking-in position (i. e., the bonding tool bounds) in some cases. CSP's (chip scale packages) may be cited as an example of such semiconductor devices.
In such a CSP, as shown in FIG. 5, a plastic insulating sheet 6 is bonded to a tape 5 on which a wiring pattern 4 is formed, and a semiconductor chip 8 is fastened to the surface of this plastic insulating sheet 6 via a paste 7. If the paste 7 is applied directly to the wiring pattern 4 on the tape 5, since the paste 7 is conductive, the wiring pattern 4 will short out. Accordingly, an insulating sheet 6 is bonded between the tape 5 and paste 7.
Such insulating sheets 6 include thermosetting sheets. However, since the insulating sheet 6 is generally softer than the cured paste 7, some sinking-in of the bonding points on the semiconductor chip 8 occurs when the bonding tool presses the semiconductor chip during bonding. Furthermore, in order to prevent the paste 7 from being squeezed out onto the tape 5, the paste 7 is applied in an area that is smaller than the semiconductor chip 8; however, such a paste 7 forms gaps 12 at the ends of the semiconductor chip 8, thus causing the sinking-in to occur.
FIG. 6 shows the sinking-in positions Sn and amounts of sinking-in .DELTA.Sn of the bonding points Pn caused by the pressing of the bonding tool 10 when the bonding tool 10 bonds the wire to the bonding points Pn on the semiconductor chip 8. In FIG. 6, n indicates the number of bonding points (natural number).
FIG. 7 shows how an insulating sheet 6 is present on the semiconductor device 1 in conventional wire bonding, and how deformation of the insulating sheet 6 caused by the pressing of the bonding tool 10 is translated into the amount of sinking-in .DELTA.Sn of the bonding points Pn on the semiconductor chip 8.
FIG. 7(a) shows the sinking-in of a bonding point P.sub.1 located on the central portion of the semiconductor chip 8 during bonding to said bonding point P.sub.1, FIG. 7(b) shows the sinking-in of a bonding point P.sub.2 located on the end portion of the semiconductor chip 8 during bonding to the bonding point P.sub.2, and FIG. 7(c) shows the raising of the bonding tool 10 to the intermediate positions C.sub.1 and C.sub.2 that correspond to the intermediate position C in FIG. 4 in the respective cases of FIGS. 7(a) and 7(b).
Furthermore, S.sub.0 indicated by a two-dot chain line represents the bonding position of the bonding tool 10 at the respective bonding points Pn after the bonding tool 10 has bonded the wire to the bonding points Pn of the semiconductor chip 8. The amounts of sinking-in .DELTA.S.sub.1, and .DELTA.S.sub.2 are exaggerated in FIG. 7 in order to facilitate understanding. The amount of sinking-in .DELTA.S.sub.1, is approximately 5 microns (.mu.m), and the amount of sinking-in .DELTA.S.sub.2 is approximately 25 microns.
The amount of sinking-in .DELTA.S.sub.2 of the end-portion bonding point P.sub.2 shown in FIG. 7(b) is larger than the amount of sinking-in .DELTA.S.sub.1, of the central-portion bonding point P.sub.1 shown in FIG. 7(a). As shown in FIG. 7(c), when the bonding tool 10 is raised so that the pressing force exerted on the bonding points Pn by the bonding tool 10 is eliminated, the Z position of the respective bonding points Pn returns to the original Z position S.sub.0. Here, in the case of FIG. 7(a) in which the bonding point P.sub.1 is located on the central portion of the semiconductor chip 8, the bonding tool 10 is raised by a fixed amount .DELTA.H.sub.0 from the sinking-in Z position S.sub.1 and positioned at the intermediate position C.sub.1 ; and in the case of FIG. 7(b) where the bonding point P.sub.2 is located on the end portion of the semiconductor chip 8, the bonding tool 10 is raised by a fixed amount .DELTA.H.sub.0 from the sinking-in Z position S.sub.2 and positioned at the intermediate position C.sub.2. In other words, in the case of FIG. 7(a), the bonding tool 10 is raised by .DELTA.H.sub.1, which is lower than the original Z position S.sub.0 by an amount corresponding to the sinking-in amount .DELTA.S.sub.1 ; and in the case of FIG. 7(b), the bonding tool 10 is raised by .DELTA.H.sub.2, which is lower than the original Z position S.sub.0 by an amount corresponding to the sinking-in amount .DELTA.S.sub.2. Accordingly, in the case of FIG. 7(b), the amount by which the bonding tool 10 is raised is smaller than in the case of FIG. 7(a) by .DELTA.S=.DELTA.S.sub.2 -.DELTA.S.sub.1.
Furthermore, in cases where no insulating sheet 6 is provided, as shown in FIG. 8, if the tape 5 is pliable, and the paste 7 is applied in an area smaller than the semiconductor chip 8, then the problems as described above occur. More specifically, when bonding is performed on the central-portion point P.sub.1. shown in FIG. 8(a), the amount of sinking-in .DELTA.S.sub.1, is the same as in a case where an insulating sheet 6 is used. However, when bonding is performed on the end-portion bonding point P.sub.2 shown in FIG. 8(b), since a gap 12 exists (since no paste 7 is used beneath the semiconductor chip 8), the portion 5a of the tape 5 located on the opposite side from the bonding tool 10 is lifted by the pressing action of the bonding tool 10, so that an amount of sinking-in .DELTA.St is generated.
Accordingly, when the bonding tool 10 is raised after bonding, and the application of pressure is eliminated as shown in FIG. 8(c), the amount by which the bonding tool 10 is raised is smaller in the case of FIG. 8(b) than in the case of FIG. 8(a) by a sinking-in amount .DELTA.St.
Furthermore, as shown in FIG. 9 as another case, if a semiconductor device is structured so that a semiconductor chip 8 is fastened to the surface of a substrate 15 via paste 7, the substrate 15 deforms when it is heated by a heating block 14 that is equipped with a heater 13, etc. As a result, when the bonding tool 10 applies pressure to the end-portion bonding point P.sub.2 on such a semiconductor chip 8, a sinking-in amount .DELTA.Ss is generated because of the deformed portion 15a of the substrate 15. In this case as well, the amount by which the bonding tool 10 is raised following bonding is reduced by the sinking-in amount .DELTA.Ss.
As seen from the above, the Z position of the bonding tool when the bonding tool bonds the ball to the bonding point is used as a reference position, and the bonding tool is raised after the raising amount of the bonding tool is calculated based upon this reference position and looping parameters stored in the memory beforehand. Accordingly, the height of the loop and the loop shape tend to differ for each bonding point.